This invention relates in general to calibrating scanners and, more particularly, to calibrating a transparency scanner using a light attenuating filter.
Many scanners are able to scan transparencies such as slides, and negatives. A light source or lamp is positioned on one side of the transparency to be scanned and illuminates the transparency. A light sensor is positioned directly opposite the light source to receive the illumination after it passes through the transparency.
Typically, the scanning process includes a calibration step before the transparency is scanned. In the calibration step, the light sensor and the light source are moved to an unobstructed area where light passes directly from the light source to the light sensor without passing through the transparency. An image of the light source is then captured by the light sensor. Operating parameters of the scanner are then adjusted so that the image of the light source captured by the light sensor is interpreted by the scanner as full exposure. Full exposure translates into full white in an image created by the scan. Factors that may influence the image captured by the light sensor include lamp brightness, lamp color, lamp illumination profile, and light sensor pixel response.
The media used for most transparencies attenuates light by a significant amount. Even in the lightest area, the transparency significantly attenuates light from the scanner""s light source. After a conventional calibration, the brightest area of the transparency during a scan will be significantly lower than full exposure. Therefore, scanning a transparency after a conventional scan will result in an image that is darker than full white.
One prior solution attempts to compensate for the light attenuation by increasing the exposure or intensity of the light source. However, the prior art does not present a solution for determining by what amount the exposure or intensity of the light source should be increased.
According to principles of the present invention, a light attenuating filter is introduced between a light source and a light sensor in a calibration region of a transparency scanner. Light from the light source is directed through the light attenuating filter and onto the light sensor. The transparency scanner is calibrated to compensate for the attenuation of light by the light attenuating filter.
According to further principles of the present invention, the transparency scanner includes a carriage for supporting the light source, a transparent platen, and a housing for supporting the transparent platen. The light attenuating filter is placed in the calibration region either as part of a template resting on the transparent platen, as part of housing, or as part of the carriage.
According to further principles of the present invention, calibrating the transparency scanner includes adjusting one or more parameters for controlling the transparency scanner. Examples of parameters used for controlling the transparency scanner include light source intensity, light source exposure time, and analog and digital gain of a signal produced by the light sensor.
According to further principles of the present invention, in order to achieve a color balance in the calibration, the light attenuating filter may attenuate light of one color more than light of another color. When the transparency scanner is calibrated using this filter, color balance of the transparency scanner is adjusted.
Other objects, advantages, and capabilities of the present invention will become more apparent as the description proceeds.